[media] cx231xx: use core-assisted lock

Instead of doing its own lock, use core-assisted one. As a bonus, it
will do the proper unlock during queue wait events. This fixes a
long-standing bug where softwares like tvtime would hang if you try to
use cx231xx-alsa.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>